(1) Field of the Invention
The invention relates to digital time-domain beamforming and more particularly to an improved time-interpolation method for digital time-domain beamformers.
(2) Description of the Prior Art
Beamforming is the process of combining input values from spatially separated receiving elements (which form an array of input channels) such that the output responds most strongly to signals arriving from a specific direction. Typically, many beams are formed simultaneously from a set of input channels with each beam pointing in a different direction. The processing associated with a single beam will be referred to herein as a beam process. In the time domain, a beam process consists of forming a weighted sum of time delayed input channels where the time delays are determined by the look direction of the beam. In a sampled time system, the time-delayed value of an input channel is generally created by interpolating between the nearest time sampled values to generate an estimate of the signal that existed at the desired delay time. The time delay of the input channel is thus composed of an integer delay (i.e., the number of sample intervals to the nearest sample) plus an interpolated fractional delay. A detailed discussion of time-delayed beamforming can be found in "Principles of Underwater Sound for Engineers" by Robert J. Urick, McGraw Hill Publishing Company, 1975.
There are several prior art methods of generating the value of the input channel at the fractional delay when beamforming. In one method, each beam process fractionally delays each channel input. A beam process delays each channel individually by using a separate interpolation filter for each channel. This requires a processing load which is proportional to the product of the number of channels, the number of beams, and the number of coefficients in the interpolation filter. Accordingly, this method requires a large processing load which limits the number of beams which can be formed simultaneously.
In a second prior art method, each channel is pre-processed by a complete set of interpolation filters to thereby increase the sampling rate. The sampling rate is increased by a ratio sufficient to reduce the error associated with using the nearest available sample delay. This up-sampled set of channel values is then transmitted to the beam processors. Each beam process then selects the appropriate fractionally delayed value for each channel as it forms its beam. Although this second method can significantly reduce the processing load compared with the previous method, it has the disadvantage of requiring a high communication bandwidth to transmit the up-sampled channel values to the beam processors.